The present invention relates generally to object-based high level programming environments, and more particularly, to techniques for tracking references to objects defined in object-based high level programming environments.
Recently, the Java programming environment has become quite popular. The Java programming language is a language that is designed to be portable enough to be executed on a wide range of computers ranging from small devices (e.g., pagers, cell phones and smart cards) up to supercomputers. Computer programs written in the Java programming language (and other languages) may be compiled into Java Bytecode instructions that are suitable for execution by a Java virtual machine implementation.
The Java virtual machine is commonly implemented in software by means of an interpreter for the Java virtual machine instruction set but, in general, may be software, hardware, or both. A particular Java virtual machine implementation and corresponding support libraries together constitute a Java runtime environment.
Computer programs in the Java programming language are arranged in one or more classes or interfaces (referred to herein jointly as classes or class files). Such programs are generally platform, i.e., hardware and operating system, independent. As such, these computer programs may be executed without modification on any computer that is able to run an implementation of the Java runtime environment.
Object-oriented classes written in the Java programming language are compiled to a particular binary format called the “class file format.” The class file includes various components associated with a single class. These components can be, for example, methods and/or interfaces associated with the class. In addition, the class file format can include a significant amount of ancillary information that is associated with the class. The class file format (as well as the general operation of the Java virtual machine) is described in some detail in The Java Virtual Machine Specification, Second Edition, by Tim Lindholm and Frank Yellin, which is hereby incorporated herein by reference.
As an object-oriented programming language, Java utilizes the programming concept known as an object. In the Java programming environment, Java objects are created (instantiated) from Java classes. Typically, Java objects are stored in a heap memory portion (heap). To illustrate, Fig.1 depicts a computing environment 100 including a heap memory portion 102 suitable for storing Java objects. As shown in FIG. 1, various Java objects, for example, objects O1, O2, O3, O4, O5 and O6 can be stored in the heap memory portion 102. A Java object in the memory portion 102, for example, Java object O3, can include a reference to its class, as well as one or more other fields describing data (e.g., variables) associated with the object. The Java object O3 can also include references to other Java objects, for example, Java objects O4 and O5, which are also stored in the heap memory portion 102.
Java objects are typically created in the heap memory portion 102 when they are instantiated. After a Java object has been instantiated, it can be referenced from various points in the Java program. For example, the object O3 can be referenced by a local variable 104 of the Java program. During the execution time of the Java program, as depicted in FIG. 1, the local variable 104 can be on an execution stack 106 in a stack frame portion 108. The stack frame portion 108 represents the stack frame for a method associated with the local variable 104. The stack frame portion 108 is typically placed on the execution stack 106 when the associated method is invoked.
In addition to the local variables associated with the method, the stack frame portion 108 also includes an operand stack portion 110 suitable for placing various operands on the execution stack 106. In the Java programming environment, these operands are placed on the operand stack portion 110 of the execution stack 106 in order to execute the Java method associated with the stack frame 108. As is known to those skilled in the art, these operands can be references to objects stored in the heap memory portion 102, e.g., an operand 112 referencing the Java object O3. 
As is known to those skilled in the art, there may be a need to identify and track references to Java objects for various reasons. For example, during the course of the execution of Java programs, some of the objects in the heap memory portion 102 are no longer needed (i.e., become “dead objects” which are no longer reachable by the Java program). Accordingly, it is desirable to identify the “dead” objects in the heap memory portion 102 and remove them from the heap. This operation can be referred to as “garbage collection.”
As noted above, entries of the execution stack can be references to Java objects stored in the heap memory portion. Therefore, to perform garbage collection for Java programs, there is a need to identify entries on the execution stack that are references to objects stored on the heap memory portion. The conservative approach to garbage collection would require traversing the execution stack and identifying every entry on the stack that could potentially be a reference to an object in the heap. Unfortunately, this conservative approach typically results in identifying dead objects as live objects. Since an object in the heap may refer to other objects, identifying a dead object as a live one can seriously hinder garbage collection.
Another approach to garbage collection seeks to identify references to live objects more accurately. However, this approach requires use of another interpreter, namely, the abstract interpreter. The abstract interpreter essentially simulates the execution of Java methods and operates when the main interpreter is suspended. Thus, the use of an abstract interpreter can adversely effect performance of Java programs. Moreover, devoting memory space and execution time to use an interpreter is not a feasible method for computing systems with relatively limited resources (e.g., embedded systems with relatively smaller memory and computing power).
In view of the foregoing, there is a need for improved techniques for tracking and identifying references to Java objects.